In a lighting system having several individual light sources which are capable of communicating with a remote controller, a desired control feature is to be able to control the light output of an individual light source merely by pointing at it with the remote controller and operating a control mechanism, such as buttons or the like.
However, in order to make this work, the remote controller has to be able to identify which one of the light sources the user is actually pointing at. Methods have been developed where each light source transmits a different code in a directional signal by means of modulating its ordinary light output or by means of modulating a separate code transmitting element, such as an IR-LED (InfraRed Light Emitting Diode) or a radio frequency transmitter, e.g. a 60 GHz directional transmitter. The code most prominently received, according to some criterion, by the remote controller is selected. For example the criterion can be “smallest angle of incidence” or “strongest optical signal”, etc.
For example, the publication WO 2007/095740 discloses a lighting system where each light source is configured to emit a beacon signal representative of the unique identifier, i.e. code, thereof on command of a remote controller. That is, the remote controller transmits an instruction to the light source that commands the light source to transmit the beacon signal, which is a directional signal. The beacon signal is integrated into the light emitted by the ordinary light source. The remote controller is configured to receive the light and extract the beacon signal therefrom. There are problems with such a lighting system.
One problem is related to synchronization. The remote controller commands several light sources to transmit their codes at the same time. In order for the remote controller to be able to separate the received codes from each other it is equipped with circuitry for correlating the optical signals received from different light sources in one way or the other. In order to obtain a reliable result in determining which light source is the most prominent one, it is desirable that the optical signals are received by the remote controller at an anticipated point of time, and substantially simultaneous.
There exist several sources of time differentiation. Inter alia, there is a variance in the time it takes for the instruction to be generated by the remote controller and actually leave its transmitter. For example the processing of the instruction can be interrupted by other processes in the remote controller. Furthermore, there is a variance in the time the transmitter of the remote controller has to wait to put the instruction into the air. Most popular current wireless transmission systems for remote control are built on the IEEE.802.15.4 standard, such as the ZigBee standard, which employs CSMA-CA (Carrier Sense Multiple Access—Collision Avoidance). In this form of multiple access, the transmitter has to wait for other transmissions to finish before putting its own message in the air. This is called “back-off”. Whether there will be none, one or several back-offs is unknown at the time of instruction generation. These variances cause undesired jitter in the time that the codes are actually detected by the remote controller.